The present invention pertains to the field of conduit extrusion, particularly for the manufacture of flexible, liquid-tight conduit of the type used in electrical applications, e.g. between junctions and other such components. In a conventional manufacturing process, it is typical to manufacture conduit by coextruding flexible and rigid PVC and wrapping the coextrusion around a winding mandrel. As the winding mandrel rotates, the coextrusion advances therealong to produce a helically wound conduit.
The flexible PVC is molten upon extrusion, and so the adjacent edges of the helically-wound conduit become self-welded to produce a continuous tube with a helical seam. This seam is a region of mechanical weakness, and it is common for the weld to fail along the seam, causing the conduit to "unwrap." The strength of the weld is very sensitive to the grade of PVC material used, and the best results are only obtained by using very high-quality material which contributes significantly to the cost of the product.
During the extrusion step of this process, the extrusion head remains stationary and the conduit rotates along with the winding mandrel. A rotating conduit is not easily coiled or otherwise accumulated, since such would twist the conduit. Thus, the final length of the conduit is limited to what can be taken straight off the mandrel, placing an undesirable length restriction on the conduit product.
In an alternative previous process, a stationary calibration rod is used with a rotating extruder head, which rotates to deposit the extrusion on the stationary calibration rod. The calibration rod assembly is hollow and includes a plurality of "tapes," i.e. continuous belts configured around the peripheral exterior of the calibration rod. The belts are threaded longitudinally from the inside around to the outside. The belts are pulled by a conventional haul-off arrangement as the extrusion is applied, which longitudinally advances the conduit, resulting in a helical winding. In this way, a continuous conduit can be made having no particular limitation on length.
However, with this method, there are small gaps between adjacent belts, and these gaps produce small interior notches on the inner wall of the conduit. These notches represent lines of weakness, along which the conduits can fail. Also, during manufacture, the tapes wear out very quickly, and break after only a few hours of use, thus requiring continuous intervals of maintenance and costly down-time.
Flexible electrical conduit must conform with the standards prescribed by Underwriters Laboratories in UL 1660. As indicated in Section 10 of UL 1660, conduit must pass a tension test in which a 300 lb. weight hangs from the end of a 44 inch length of conduit for a 60 second period without opening or uncoiling. Such conduit must also withstand an impact test, in which a weight is dropped on the conduit, which must resist breaking. The UL has recently harmonized standards with those of the Canadian Standards Association (CSA 22.2). All electrical conduit must now satisfy a cold weather impact test in which nine ft. lbs. of energy is impacted to a six inch length of conduit at a temperature of -18.degree. C. (For example, a 9 lb. weight dropped from a height of one ft., etc.) The conduit made by the previous processes broke along the notches or weld seams and therefore failed the test.